Every problem on this sheet has two different denominators, so none of them can be added straight across - a common denominator has to be found first, every time. That is the point of drilling it this way: the procedure gets repeated until it is automatic.
It is also the limitation. A page where every problem needs a common denominator does not ask the student to notice when one is needed. Once the mechanic is reliable, move to the Adding Fractions page, which mixes both kinds and puts that decision back.
Multiplying the two denominators together always gives one, and the answer reduces at the end anyway. The least common multiple gives smaller numbers to work with if a student can find it.
In lowest terms, and as a mixed number rather than an improper fraction - so 6/4 is given as 1 1/2 and 8/4 as 2. Turn on Include Answer Pages to print the key behind the sheet.
Fifth grade in most curricula - adding and subtracting fractions with unlike denominators is one of that year's core fraction standards.
Set Largest Denominator to 6. Smaller denominators mean smaller common denominators and less reducing at the end.